Multiple catalyst beds

Schematic representation of a typical two-stage hydroconversion unit. The reactors contain multiple catalyst beds and quench zones. The second stage reactor is a recycle reactor.

Use of multiple beds/reactors in series with intercooling or quenching is a method which can be used to reduce the amount of recycle and its associated costs. Multiple catalyst beds reduce costs by using the recycle material several times before it is separated from the reaction products. 

Fig. 7.5. Heatup path for gas descending the Fig. 7.1 catalyst bed. It begins at the feed gas s input temperature and 0% SO2 oxidized. Its temperature rises as S02 oxidizes. Maximum attainable S02 oxidation is predicted by the heatup path-equilibrium curve intercept, 69% oxidized at 893 K in this case. This low % SO> oxidized confirms that efficient SO-> oxidation cannot be obtained in a single catalyst bed. Multiple catalyst beds with gas cooling between must be used.

The second, currently in operation, allows for unit production capacities of 1500t/day, in vertical reactors with multiple catalyst beds (normally two) usually operating wiA axial flow, at a pressure of 20 to 25. 10 Pa absolute. Cooling systems are of two types ... 

A typical EBOne plant flow diagram is shown in Fig. 4. The alkylation reactor is maintained in the liquid phase and uses multiple catalyst beds and ethylene injections to improve the reaction selectivity. Dividing the ethylene into multiple feed streams keeps the alkylation catalyst deactivation rate very low. In some plants using EBZ-500 catalyst, operating lengths of more than 8 yr have been obtained without catalyst regeneration. The ethylene conversion is essentially 100% in the alkylation reactors, and the reactors operate nearly adiabatically. The exothermic heat of reaction is recovered and used within the process to heat internal process streams or to generate steam. 

Fig. 2 shows the I.C.I. warm-shot methanol synthesis loop. The adiabatic methanol reactor has multiple catalyst beds which cure quenched with warm reactant gas that control the methanol converter s temperature profile and methanol outlet concentration as portrayed in Fig. 3.. In this adiabatic quench redctor methanol loop scheme, the main features are ...

In the early days the synthesis gas was produced at atmospheric pressure, and the synthesis gas was compressed in reciprocating compressors to pressures as high as 100 MPa in some cases. Capacities were limited to around 300 - 400 MTPD due to limitations in reciprocating compressors. However, with the development of steam reformer based front-ends and the introduction of centrifugal compressors, the ammonia plant capacities suddenly increased to 1000 MTPD with ammonia synthesis loop pressures typically around 15 MPa. Since the 1960 s new developments have been in the ammonia converter designs, such as introduction of radial flow converters and introduction of converters with multiple catalyst beds to increase ammonia conversion. 

In both cases the pyrolysis gasoline/gas oil feed enters at the top of the first reactor. A recycle stream of hydrotreated gas oil is injected with the feed. The recycle streams serve as a reactant diluent, a heat sink to aid in reactor temperature control and as a solvent for polymer removal. Multiple catalyst beds are employed in the reactors to aid in temperature control. Quench oil is injected between the beds for reaction heat control. The first stage is operated at temperatures in the range of 107-177 C and at hydrogen partial pressures of the order of 48-68 atmospheres. 

For the design of a reactor with multiple catalyst beds and intermediate cooling, knowledge of the reaction rate of SO2 oxidation is essential. According to literature, the oxidation rate of SO2 on a typical industrial V2O5 catalyst is given by (Froment and Bischoff, 1990) ... 

Figure 6.11.28 Flow sheet of methanol synthesis in an adiabatic quench reactor with multiple catalyst beds. Figure adapted from (Baerns et al. 2006).

A quasi-isothermal reactor forms the basis of the BAYQIK (guasi-/sotherme-ATatalyse) process developed by Bayer Technology Services (Weber et al., 2011). The reactor is designed so that the heat of reaction generated by SO2 oxidation can be removed as soon as it is produced, eliminating the need for multiple catalyst beds and gas heat exchangers. 

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